The mesengenic process. Adult mesenchymal stem cells (MSCs) are able to differentiate into bone, cartilage, muscle, marrow stroma, tendon/ligament, fat and other connective tissues in a sequence of lineage transitions. This figure was first drawn to mirror the sequence of events observed in haematopoietic differentiation. The state of knowledge in the late 1980s and early 1990s provided the most information for the lineages on the left and the least information for the pathways on the right. (This information is reviewed in [1–3])